Unlike traditional spectrum sharing model with a predetermined fixed interference threshold at the primary receiver, in the paper we consider a new spectrum sharing model with a elastic controllable interference varying between 0 and bottom threshold that is maximum the primary receiver can tolerated, and we introduce a seller's market trading mechanism in order to fully exploit the interference room, as has very important practical significance. In the context, we investigate the secondary user's (SU's) maximal gross profit and optimal power allocation scheme in Rayleigh fading environment, and also study the interference price and average revenue of SU's impact on them. Numerical calculation results are conducted to verify our theoretical results.